Semiconductor light-emitting elements such as a light-emitting diode (LED) and the like have advantages that they are small, have a little power consumption, and are able to stably perform high brightness light emission, and in recent years, a movement is ongoing to replace illumination devices such as an incandescent light and the like with illumination devices using a light-emitting device that emits white light and includes an LED. As the LED that emits the white light, for example, there is a combination of a blue LED and a Ce-activated. YAG fluorescent material that is indicated by a composition formula of (Y, Gd)3(Al, Ga)5O12:Ce.
In the light-emitting device having the above structure, the white light is achieved by mixing of blue light from the LED and yellow light from the Ce-activated YAG fluorescent material as the fluorescent material. In this structure, a red color component is deficient because of a light emission property of the Ce-activated YAG fluorescent material, and in a case of being used in a home illumination device and the like, for example, a disadvantage that human skin colors look unnaturally and the like occur.
Specifically, in the above light-emitting device, in a color temperature region defined by a neutral white color and a warm white color that are used in the illumination device, an average color rendering evaluation number (hereinafter, called Ra) is about 70 to about 75, and a special color rendering evaluation number (hereinafter, called R9) indicating how the red color looks is about −40 to about −5, accordingly, when used as an illumination device, the red color looks extremely poorly.
Because of this, to improve the above color rendering property such as Ra, R9 and the like, a structure and the like are proposed, in which besides the above blue LED, a yellow fluorescent material such as a YAG fluorescent material and the like, a green fluorescent material and a red fluorescent material of a nitride relative and the like are combined. Under the circumstances, a patent document 1 discloses a white light-emitting device that uses a blue LED as an excitation light source and is obtained by combining an orange fluorescent material and a green fluorescent material that have a light emission wavelength of 560 to 590 nm as a combination that has both high color rendering property and stability. Although not as a specific example of the white light-emitting device obtained by combining fluorescent materials, this document discloses an α SiAlON fluorescent material and a β SiAlON fluorescent material as examples of the orange fluorescent material and the green fluorescent material, respectively.
A patent document 2 discloses and proposes a combination of an Eu-activated α SiAlON fluorescent material as the yellow fluorescent material, an Eu-activated β SiAlON fluorescent material as the green fluorescent material, and an Eu-activated CaAlSiN3 fluorescent material as the red fluorescent material.
Besides, a non-patent document 1 shows a relationship between: Ra of a white LED obtained by combining a fluorescent material and a blue LED; and a theoretical limit of luminous efficacy that indicates a theoretical limit of luminous efficiency of a light-emitting device. A non-patent document 2 discloses a method that is described in the non-patent document 1 and used to measure internal quantum efficiency of a fluorescent material, and a non-patent document 3 discloses Eu-activated SrAlSiN3 as an example of another kind of fluorescent material that has a composition different from the fluorescent material disclosed in the present application.